The invention relates to a method of adjusting an excitation and detection circuit for nuclear magnetic resonance (NMR) serving to improve the signal-to-noise ratio of a signal detected by said circuit.
The invention also relates to an NMR excitation and detection circuit adapted to enable such a method to be implemented.
The invention applies to circuits including a probe of the type having a single coil for emitting a pulse for exciting the nuclear spins of a sample immersed in a magnetic field and for detecting a resonance signal from said nuclear spins. Such probes are used mainly in NMR spectroscopy, but sometimes also in nuclear magnetic resonance imaging (MRI).
Nuclear magnetic resonance or NMR is a method of performing precise chemical analysis of matter, but it suffers from a lack of sensitivity due to the small energies involved. Any method that makes it possible to increase this sensitivity will serve to extend the field of application of NMR and its best known branch MRI.
The principle of nuclear magnetic resonance by Fourier transform, which is the method that is presently the most sensitive and the most widespread, consists in placing the sample in an external static magnetic field B0 and in exciting nuclear magnetization M by means of an electromagnetic pulse at radiofrequency of amplitude B1 perpendicular to the static magnetic field B0 and at a frequency that is close to the nuclear spin resonant frequency:
      f    0    =                          γ        ⁢                                  ⁢                  B          0                                  2      ⁢                          ⁢      π      known as the Larmor frequency, where γ is the gyromagnetic ratio.
It is important to observe that since the value of the static magnetic field B0 in most experimental and commercial setups is constant, and since the gyromagnetic ratio of an isotope under consideration varies very little (relative variation less than 10−4 and typically about 10−5) depending on the chemical environment of its atom, that amounts to accepting that the Larmor frequency is a constant that is determined by the experimental setup.
After excitation, the nuclear magnetization describes a precession movement around the axis defined by B0 at its Larmor frequency. This precession of the magnetization acts by the Faraday effect to create a current in a coil of axis perpendicular to B0, which current corresponds to the signal that is actually detected. In practice, since the signal is weak, it is initially amplified before being multiplied by a reference frequency, typically that of the excitation radiofrequency (RF) field. By conserving only the low frequency portion of the signal (lowpass filter), the time-domain signal is subsequently converted digitally, and after applying a Fourier transform the spectrum is deduced therefrom. It can thus be seen that it is necessary to have beside the coil that creates the magnetic field B0, at least one other coil that creates a radiofrequency field B1 perpendicular to B0. In most configurations, because of problems associated with space and crossing difficulties between the radiofrequency coils, it is found simpler to use only one radiofrequency coil, which coil is used both for excitation and for detection. That one coil is associated with capacitive elements so as to constitute a resonant circuit tuned to the Larmor frequency of the signal emitted by the nuclear spins of the atoms to be detected. An electronic circuit, often made of two diodes connected head to tail in parallel, placed between said resonant circuit and the preamplifier, serves to separate the transmission circuit from the reception circuit while protecting the reception circuit and in particular the preamplifier from voltage surges that are present during excitation [1]. At the coil, by tuning the resonant frequency fr of the resonant frequency containing the coil to the Larmor frequency f0, an improvement is obtained in NMR detection sensitivity by a factor of Q1/2 with:
  Q  =            2      ⁢                          ⁢      π      ⁢                          ⁢              Lf        r              r  where L is the inductance of the coil and r is its resistance. To do this, variable capacitors are added in series and in parallel. The electronic circuit comprising the coil and the tuning capacitors is referred to as a measurement head or “probe” and it constitutes the main portion of the excitation and detection circuit. It is also necessary to match impedance. That consists in matching the impedance of the probe to the impedance of the amplifier of the transmitter circuit, amongst other things, so that the power delivered thereby is not reflected. Matching is generally performed at 50 ohms (Ω).
In practice, because of the high Larmor frequency associated with the static magnetic field, i.e. frequencies of several tens to several hundreds of megahertz, the tuning circuit of the probe is a reactive circuit which generally comprises a tuning capacitor represented by the value Ct of its capacitance, generally connected in parallel with the coil, having the main purpose of adjusting the resonant frequency fr of the electronic circuit to the Larmor frequency f0 of the nuclear spins of the atoms that are to be detected, and a matching capacitor represented by the value Cm of its capacitance, generally connected in series with the parallel connection of the coil and the tuning capacitance Ct, for the purpose of matching the impedance of the probe to 50Ω. The adjustment of the values of the adjustable capacitors, i.e. of the tuning capacitance Ct and the matching capacitance Cm, which adjustment depends on the electrical losses in the sample, is generally determined experimentally either by studying the response of the resonant circuit containing the coil, which is done with the help of an LC balun circuit connected to a wobulator, or by minimizing the signal reflected to the amplifier. Under all circumstances, the resonant circuit of the probe is tuned in transmission to the Larmor frequency of the nuclear spins of the atoms to be detected.
Normally, it is considered that the tuning of the resonant circuit of the probe in transmission suffices to obtain satisfactory tuning of said resonant circuit in reception. In other words, it is considered that the resonant frequencies in transmission and in reception of the NMR probe are substantially the same.
However, the inventors have found that when the resonant frequency of the probe is accurately tuned in transmission to the Larmor frequency of the nuclear magnetization of the atoms for detection, the same probe is not accurately tuned in reception. This tuning offset is due to various factors, such as the length of cables, parasitic capacitances of diodes, etc. Contrary to the common opinion in the art of NMR spectroscopy, the inventors have realized that this tuning offset is far from being negligible, and can be evaluated as several tens or even several hundreds of kilohertz for most, if not all, NMR probes and commercial setups. Such a tuning offset gives rise to a drop in the power of the received signal and to a degradation in its signal-to-noise ratio that is of the order of 1 decibel (dB) or more, which is considerable.